1. Technical Field
The present invention relates to an antireflective film, a polarizing plate, a cover glass, an image display device, and a method of manufacturing an antireflective film.
2. Background Art
In an image display device such as a cathode ray tube display (CRT), a plasma display (PDP), an electroluminescence display (ELD), a fluorescent display (VFD), a field emission display (FED), and a liquid crystal display (LCD), an antireflective film is provided in order to prevent contrast reduction or glare of the image due to the reflection of external lights on the surface of the display.
As the antireflective film, an antireflective film has been known, which includes, on a substrate surface, a fine unevenness shape in which a period is shorter than the wavelength of the visible light, that is, an antireflective layer having a so-called moth-eye structure. By the moth-eye structure, a refractive index gradient layer whose refractive index is varied continuously from the air towards a bulk material inside the substrate is artificially produced, so that reflection of light may be prevented.
As an antireflective film having the moth-eye structure, Japanese Patent Laid-Open Publication No. 2009-139796 (hereinafter JP-A-2009-139796) discloses an antireflective film having a moth-eye structure formed by coating a coating liquid containing a transparent resin monomer and fine particles on a transparent substrate, curing the coating liquid to form a transparent resin dispersed with the fine particles, and then, etching the transparent resin.
Japanese Patent Laid-Open Publication No. 2011-002759 (hereinafter JP-A-2011-002759) discloses an antireflective film having a moth-eye structure obtained by polymerizing a (meth)acrylic polymerizable composition containing a urethane (meth)acrylate and an ester (meth)acrylate.
Japanese Patent Laid-Open Publication No. 2006-145736 (hereinafter JP-A-2006-145736) discloses an antiglare-antireflective film including an antiglare layer having a uneven shape on a transparent substrate and a low refractive index layer having hollow silica particle on the antiglare layer. Such an antiglare-antireflective film, which includes, at the outermost surface, a low refractive index layer, which is a thin layer with a thickness of 200 nm or less, may prevent reflection of light by optical interference of the low refractive index layer and also improve strength of the outermost layer.
However, in JP-A-2009-139796 and JP-A-2011-002759, it is necessary to etch a transparent resin or forming a transparent resin by molding, a manufacturing process for an antireflective film is complicated in some cases. Also, in JP-A-2011-002759, a hard material is not applicable, and therefore scratch resistance becomes insufficient in some cases.
JP-A-2009-139796 also has a problem that a sufficient hardness cannot be obtained, in addition to the above problem in manufacturing process. An antireflective film disclosed in JP-A-2006-145736 does not have a moth-eye structure, and is required to further improve antireflective property.
An objective of the present invention is to provide an antireflective film having a low reflectance, no white turbidity, a high hardness, and an excellent scratch resistance, and a simple method of manufacturing such an antireflective film. Another objective of the present invention is to provide a polarizing plate, a cover glass and an image display device which include the antireflective film.
The present inventors have studied formation of a moth-eye structure without etching or forming by molding. Although they tried to form a layer having a moth-eye structure simply by coating, on a substrate, a composition (coating liquid) containing particles, and a binder resin or a binder resin forming compound, and curing the composition, but they have found the following problems (1) and (2).
(1) When a content of particles is high and a content of a binder resin or a binder resin forming compound is low before the coating, until completion of forming a film, the particles are aggregated, and a fine unevenness structure (moth-eye structure) cannot be formed. As a result, an antireflection property may disappear. Also, the particles are excessively aggregated, the aggregated portion functions as a scatter so that a film becomes white turbidity and a haze occurs. As a result, a contrast is deteriorated and a beautiful black tightness of a display due to an antireflective film is impaired.
(2) When a content of particles is low and a content of a binder resin or a binder resin forming compound is high after the coating, the particles are embedded into the binder resin or the binder resin forming compound so that an uneven shape is not formed and sufficient antireflective property is not obtained.
As a result of intensive study of the present inventors, it has been found that: when a low molecule monomer which easily permeates into a substrate or an under layer and a compound (viscosity increasing compound) which tends to prevent aggregation of particles are contained into a coating liquid for forming an antireflective layer, aggregation of particles may be prevented; by such a configuration, when a formation of a film is completed, a height of a convex portion and a distance between convex portions in an unevenness structure formed by the particles due to infiltration of the low molecule monomer can be suitably adjusted; and as a result, a moth-eye structure, which has a low reflective property, excellent uniformity on the surface, and excellent scratch resistance, is easily formed.
When infiltration of the low molecular monomer occurs, a content ratio of particles in a coating film becomes relatively high, and therefore if a viscosity increasing compound is not contained, the particles may be aggregated in the time between the coating a coating liquid and the completion of forming of a film. By viscosity-increasing effect of the viscosity increasing compound, aggregation of the particles is prevented, and even if the content ratio of particles in a coating film becomes relatively high, the particles are protruded from the film without aggregation of the particles and a desired unevenness structure may be obtained.
This effect cannot be obtained simply by heightening a solid concentration of a coating liquid for forming an antireflective film and by heightening viscosity of the coating liquid. This reason is presumed that after coating the coating liquid, a solvent is volatilized by drying, the low molecular monomer continuously permeates into the substrate or the under layer, and a content ratio of the particles in the coating film becomes relatively high and as a result, aggregation of particles cannot be prevented.
Further, the present inventors have studied that a moth-eye structure is formed without performing etching and that a high hardness is imparted.
The present inventors formed an antireflective film having a hard coat layer on a substrate and an antireflective layer having a moth-eye structure by coating, on the hard coat layer, a composition for forming an antireflective layer, containing particles and a binder resin forming compound, and curing the composition, and have found that when affinity of the binder resin forming compound with the particles is excessively high, a binder resin is filled into gaps between the particles, and a moth-eye structure is not formed and a reflectance becomes high. The present inventors have also found that when wettability of the binder resin forming compound with the particles is low, cissing occurs and a binder resin in the antireflective layer is not uniformly formed, and a reflectance becomes high.
That is, it is found that even when an antireflective layer having a moth-eye structure is simply formed on a hard coat layer, a reflectance becomes high.
The present inventors have further studied these problems and have found that these problems can be solved by using an antireflective layer on a hard coat layer, the antireflective layer having a moth-eye structure, wherein with respect to the particle constituting the moth-eye structure, an average primary particle diameter, a particle occupancy, an average value and a standard deviation of distance of a convex portion and a concave portion are suitably controlled.
That is, the above problems are solved by the following configurations.